Dynamic vulcanizates are known in the prior art, both in rubber/plastic blends, and in rubber/rubber dynamic vulcanizates. For example, U.S. Pat. No. 5,053,450 teaches dynamic vulcanization of acrylate copolymer elastomers in a matrix of fluoroelastomer, but no prior art patent suggests that one grade of fluoroelastomer might usefully be dynamically vulcanized while being blended with a second type of fluoroelastomer.
A key raw material used in some of the dynamic vulcanizates of this invention are the iodine-terminated peroxide-curable FKMs of U.S. Pat. No. 4,158,678. These polymers have iodine groups only on the chain termini (i.e., they are xe2x80x9ctelechelicxe2x80x9d polymers). The reactivity of the iodine terminal groups is very high, so that substantially all of them can be incorporated into crosslinks. These polymers have the property that, provided there is enough peroxide and coagent to cause the reaction of all the iodine-functional endgroups into the elastomer network, adding more peroxide and coagent has little additional effect.
It has been found that elastomeric polymer blends that include:
1. a first portion of one or more fluoroelastomers, known collectively as fluoroelastomer phase 1, which is readily crosslinkable by a cure system 1 that does not crosslink fluoroelastomer phase 2 (or which crosslinks as fluoroelastomer phase 2 at a much slower rate than fluoroelastomer phase 1), and
2. a second portion of one or more fluoroelastomers, known collectively as fluoroelastomer phase 2, which is capable of being crosslinked by a different cure system 2 than is used to crosslink fluoroelastomer phase 1,
can be dynamically cured during intensive mixing using cure system 1 under conditions where fluoroelastomer phase 1 is crosslinked but not fluoroelastomer phase 2. These dynamic vulcanizates can subsequently be mixed (at low temperature) with cure system 2 to produce a fluoroelastomer compound with one or more of these special properties:
high green strength due to the high gel content, which leads to good shape retention of extrusions and low extrusion die swell;
low tendency to blister, to the extent that some versions can be cured at atmospheric pressure without blistering;
low cost compared to similar compounds based on only one type of fluoroelastomer (in some commercially significant cases);
relatively high tear strength compared to a standard (non-dynamically cured) fluoroelastomers using the same filler system.
The invention includes polymer blends containing at least two and possibly three or more non-miscible fluoroelastomers, at least one of which is dynamically vulcanized. All the fluoroelastomers that cure during dynamic vulcanization of a blend of the present invention are said to constitute xe2x80x9cfluoroelastomer phase 1xe2x80x9d. Fluoroelastomer phase 1 can contain several different types of crosslinked fluoroelastomers. All the fluoroelastomers that do not cure during dynamic vulcanization of a blend of the present invention are said to constitute xe2x80x9cfluoroelastomer phase 2xe2x80x9d. Fluoroelastomer phase 2 can also contain several different types of flowable, non-crosslinked fluoroelastomers. These fluoroelastomer phases can in general consist of any workable blend of elastomers that have more than 17% by weight elemental fluorine, including all the known commercial classes of fluoroelastomer. This includes specifically blends of:
elastomeric copolymers of vinylidene fluoride and hexafluoropropene (xe2x80x9cFKM dipolymersxe2x80x9d herein);
elastomeric copolymers that are derived from at least three monomers, including vinylidene fluoride and hexafluoropropene which also contain polymerized residues from other monomers, such special cure site monomers, tetrafluoroethylene, ethylene and/or perfluorovinylethers. (xe2x80x9cFKM copolymersxe2x80x9d herein);
elastomeric xe2x80x9cperoxide-curable FKMxe2x80x9d refers to FKM copolymers that are derived from at least three monomers: vinylidene fluoride, hexafluoropropene, and one or more special reactive cure site monomers that confer peroxide-reactivity to the polymer. Peroxide-curable FKMs can also contain tetrafluoroethylene, ethylene, and/or perfluorovinylethers.
elastomeric copolymers of propene and tetrafluoroethylene (xe2x80x9cFEPM copolymersxe2x80x9d herein);
perfluoroelastomers (xe2x80x9cFFKM copolymersxe2x80x9d herein) of the polymethylene type having all fluoro, perfluoroalkyl, or perfluoroalkoxy substituent groups on the polymer chain; a small fraction of these groups may contain functionality to facilitate vulcanization;
elastomeric perfluoropolyethers, such as poly(perfluoropropyleneoxide) and copolymers thereof containing cure sites;
fluorosilicones, which consist of polydialkylsiloxanes in which at least 28% of the siloxane residues have at least one 3,3,3-trifluoropropyl residue attached to the silicon atom.
As will become be clear in the detailed discussion below, only certain particular mixtures of fluoroelastomers actually work to form the blends of this invention by dynamic vulcanization.
The presence of microscopic crosslinked fluoroelastomer particles (xe2x80x9cfluoroelastomer phase 1xe2x80x9d) in the presence of one or more flowable fluoroelastomers (xe2x80x9cfluoroelastomer phase 2xe2x80x9d) confers several useful properties, including greater resistance to blistering in low temperature curing, improved green strength, reduced extrusion die swell, improved tear strength, and/or improved economics. Improved economics comes about primarily because the method makes it possible to blend relatively inexpensive fluoroelastomers, such as for example copolymers of vinylidene fluoride and hexafluoropropene (xe2x80x9cFKM dipolymersxe2x80x9d) with more expensive peroxide-curable copolymers, fluorosilicone polymers, or perfluoroelastomers. The requirement for the process to work is that independent (or nearly so) cure systems must exist for fluoroelastomer phase 1 and fluoroelastomer phase 2, and also fluoroelastomer phase 1 must not be miscible with fluoroelastomer phase 2.
For purposes of this disclosure, xe2x80x9cfluoroelastomerxe2x80x9d means elastomeric copolymers containing 17% or more elemental fluorine. Fluoroelastomers include FKM, perfluoroelastomers, and fluorosilicone polymers.
For purposes of this disclosure, xe2x80x9cFKMxe2x80x9d without modifiers means elastomeric copolymers that are derived from at least two monomers, vinylidene fluoride and hexafluoropropene. FKM may also contain polymerized residues from other monomers, such as tetrafluoroethylene, ethylene, special cure site monomers, and/or perfluorovinylethers.
For purposes of this disclosure, xe2x80x9cFKM dipolymerxe2x80x9d means copolymers that are derived from only two monomers, vinylidene fluoride and hexafluoropropene, with vinylidene fluoride content from 40-70% by weight.
For purposes of this disclosure, xe2x80x9cFKM copolymerxe2x80x9d means elastomeric copolymers that are derived from at least three monomers, including vinylidene fluoride and hexafluoropropene which also contain polymerized residues from other monomers, such as tetrafluoroethylene, ethylene, and/or perfluorovinylethers. FKM dipolymers and copolymers are not readily crosslinkable by peroxides and/or peroxides+coagents.
For purposes of this disclosure, xe2x80x9cperoxide-curable FKMxe2x80x9d refers to FKM copolymers that are derived from at least three monomers: vinylidene fluoride, hexafluoropropene, and one or more special reactive cure site monomers that confer peroxide-reactivity to the polymer. Peroxide-curable FKMs can also contain tetrafluoroethylene, ethylene, and/or perfluorovinylethers. Peroxide-curable FKMs can be crosslinked by peroxide or another source of reactive free radicals, and preferably also a reactive coagent such as triallylisocyanurate (TAIC) or various other multifunctional vinyl group-containing coagents.
For purposes of this disclosure, perfluoroelastomers (also xe2x80x9cFFKMxe2x80x9d copolymersxe2x80x9d herein) are polymers of the polymethylene type having all fluoro, perfluoroalkyl, or perfluoroalkoxy substituent groups on the polymer chain; a small fraction of these groups may contain functionality to facilitate vulcanization.
For purposes of this disclosure, xe2x80x9cperfluoropolyethersxe2x80x9d refer to elastomeric fluoropolyethers which contain ether linkages in the main chain, such as poly(perfluoropropyleneoxide) and copolymers thereof with other cyclic oxiranes which may or may not contain fluorine and/or cure sites.
For purposes of this disclosure, alternating copolymers of propylene with tetrafluoroethylene are herein known as xe2x80x9cFEPM;xe2x80x9d a small fraction of special cure site monomers may also be copolymerized in FEPM or generated therein by reactive polymer processing to facilitate vulcanization. FEPM is normally cured with peroxide plus a reactive coagent.
Fluoroelastomers are generally used in applications requiring superior resistance to hydrocarbons (both lubricating oils and fuels), elevated temperatures, or both. Because of the low solubility of both organic vapors and water in FKM, FEPM, and FFKM, these particular fluoroelastomers in general have a high tendency to blister during curing, and therefore usually must be cured under pressure. Bisphenol- and diamine-cured FKMs evolve water vapor during curing. Peroxide-cured FKMs and FEPMs release low molecular weight organic byproducts of peroxide decomposition during curing. FFKMs may also release various volatile byproducts of curing, depending on what cure site monomers are employed therein.
Of the available prior art FKMs, the particular peroxide-crosslinkable FKMs with iodine containing cure site monomers can be formulated to have a low tendency to blister, primarily because they cure efficiently with very low amounts of peroxide (below 0.5% by weight). Examples of such materials include Ausimont""s Technoflon P-710 and all of Daikin""s peroxide-curable FKMs.
It has been found that some peroxide-vulcanizable FKM polymers are surprisingly much less reactive with diamine crosslinkers (such as Diak #1, hexamethylenediamine carbamate) or bisphenol cure systems compared to many non-peroxide-crosslinkable grades of FKM. The surprisingly low reactivity of certain iodine-functional peroxide-curable FKMs towards the diamine- or bisphenol-based cure system used to dynamically vulcanize fluoroelastomer phase 1 make these especially desirable as fluoroelastomer phase 2 polymers. The difference in reactivity is great enough to selectively vulcanize a general-purpose FKM (fluoroelastomer phase 1) with diamine or bisphenol while mixing with a peroxide-vulcanizable FKM grade or grades (fluoroelastomer phase 2). All such combinations that work to produce processable dynamic vulcanizates are part of the present invention. The particularly preferred combinations are those of readily diamine- or bisphenol-vulcanizable FKM elastomers as fluoroelastomer phase 1, with an iodine-functional peroxide-curable FKM as fluoroelastomer phase 2.
Table 1 gives several examples of the present invention. These particular compounds are all examples of diamine-, or bisphenol-cured FKM dipolymer or copolymer as fluoroelastomer phase 1, cured in the presence of one or more FKM copolymers which are crosslinkable by peroxide or another source of reactive free radicals (fluoroelastomer phase 2). The distinguishing feature of combinations of materials that work is that the elastomers that comprise fluoroelastomer phase 1 must be more reactive with the selected diamine-, polyamine-, bisphenol-, or polyphenol-based cure system than the elastomers that comprise fluoroelastomer phase 2. The invention is not limited to these particular dynamically vulcanized fluoroelastomer blends, but also applies to various combinations of fluoroelastomers (as described below) that are expected to work in the process of this invention to produce useful elastomeric materials.
A particularly promising set of applications for the products of this invention are extrusions (for example hoses, wire and cable insulation, and wiper blades) which are cured at elevated temperature and atmospheric pressure (or nearly so). Three particular examples of such low pressure, high temperature curing processes are hot air vulcanization (HAV), curing in molten salt baths, and microwave curing lines.
In such low pressure, high temperature curing processes, the extruded FKM profile must be processable without scorching at typical rubber extrusion conditions (xcx9c120xc2x0 C.), following which the extrudate is heated to a higher cure temperature downstream of the extruder. Whether the extrudate is heated by conduction, convection, or radiation, conventional FKM thermoset compounds of the prior art are prone to blistering when cured at ambient pressure, whereas certain dynamically vulcanized blends of the present invention are unusually resistant to blistering.
Another promising application for the compounds of this invention is for low-temperature FKMs. In this application, a high-cost, low-temperature FKM (such as Viton GLT, Technoflon P-710, or Dai-E1 LT-302) is used as the non-curing matrix phase polymer in a dynamically vulcanized blend with ordinary FKM. The effectiveness of a minor portion of a high-cost, low-TgFKM in such a blend with micron-sized crosslinked particles of ordinary FKM is enhanced because the low-Tg FKM is present as the continuous or xe2x80x9cmatrixxe2x80x9d phase. (Tg refers to the glass transition temperature, which is closely related to the temperature at which an elastomer becomes brittle.)
Another promising application for the compounds of this invention is to FKM compounds requiring improved tear strength, especially hot tear strength. It is surprising that some particular versions of the present invention have improved tear strength over similar prior art non-dynamically cured compounds that have the same volume fractions of polymers and fillers.
It is also feasible to create dynamic vulcanizates in which the peroxide-crosslinkable FKM is xe2x80x9cfluoroelastomer phase 1,xe2x80x9d and is dynamically vulcanized in a matrix of a bisphenol- or diamine-cured FKM as xe2x80x9cfluoroelastomer phase 2.xe2x80x9d However, such blends are much more expensive given the higher cost of peroxide-curable FKM, and furthermore these blends are prone to blistering. Although in most cases this reverse-mode dynamic vulcanization is less desirable, there may be certain particular instances where this reversal of curing versus non-curing FKM phases may be desirable.
Another promising application for the compounds of this invention is to create blends that are not feasible by ordinary blending methods. FEPM for example can be dynamically crosslinked in a matrix of a bisphenol-curable FKM copolymer, such as Dyneon""s Base Resistant Elastomer (BRE).
Another particularly promising application of the invention is to dynamically vulcanize fluorosilicone in a matrix of FKM. One version of this is to dynamically vulcanize fluorosilicone with peroxide in a matrix of diamine- or bisphenol-curable FKM. A particularly desirable way to dynamically vulcanize fluorosilicone in a matrix of FKM is to use platinum/hydride-vulcanized fluorosilicone as fluoroelastomer phase 1, dynamically cured in a matrix of a peroxide-curable FKM. In this case, the cure sites present on the fluorosilicone are vinyl-functional and are reactive with SiH groups in an added hydride-functional silicone fluid (which can be a standard silicone or a fluorosilicone) when catalyzed with platinum or another suitable catalyst. (The reactive groups on peroxide curable FKM do not form crosslinks with hydride-functional fluids.)
Another particularly promising application of the invention is to dynamically vulcanize diamine- or bisphenol-cured FKM in a matrix of fluorosilicone. The fluorosilicone can be peroxide-cured or it can be cured by a hydride (R3Sixe2x80x94H) functional oligomer that forms crosslinks via addition reactions with vinyl groups on the fluorosilicone. Hydride addition reactions are necessarily catalyzed, normally by platinum or rhodium, but other catalysts may also be used in principle. It is critical in this case that the FKM cure system not cause degradation of the fluorosilicone nor deactivation of the hydride addition catalyst. Some types of FKM cure systems involving strong base or phosphonium accelerators may not be workable in conjunction with the catalyzed hydride cure system for fluorosilicones, but in these cases, it is still possible to crosslink the fluorosilicone with peroxides. As in any peroxide cure, reactive coagents may also be employed.
Another application of the invention is to dynamically vulcanize a diamine- or bisphenol-cured FKM in a matrix of FEPM.
Another application of the invention is to dynamically vulcanize FKM in a matrix of FFKM. There are several different known cure mechanisms for FFKM (perfluoroelastomer). In some instances, FFKM may be non-reactive towards peroxides, in which case the peroxide-curable FKM in such a blend can be dynamically vulcanized by peroxides in the presence of the FFKM. On the other hand, FFKM that is cured by a cycloaddition reaction of acetylene groups, is also reactive with free radicals from peroxide, and so this type of FFKM will preferably be dynamically cured with bisphenol-cured FKM.